Conventionally, fluid pressure units have been known that drive an actuator by pumping fluid by a fluid pressure pump. As a fluid pressure unit of this kind, for example, Patent Document 1 discloses a hydraulic power unit. This hydraulic power unit includes a hydraulic pump as a fluid pressure pump, a hydraulic cylinder as an actuator, and a tank. The hydraulic pump is driven by a variable-speed motor to suck working oil from the tank and to pump it to the hydraulic cylinder. Whereby, the hydraulic cylinder is driven.
In the above hydraulic power unit, level lowering of the working oil in the tank may cause the hydraulic pump to suck air together with the working oil. Air can be mixed with the working oil by some other factors, which may also cause the hydraulic pump to suck the air. When the hydraulic pump is kept being driven in this state, namely, when the hydraulic pump continues a generally-called dry operation, the hydraulic pump may fall in a lubrication insufficient state to cause seizing.
To tackle this problem, it can be considered to apply dry operation detection means as disclosed in, for example, Patent Document 2 to the hydraulic power unit.
Specifically, a control device for an engine in Patent Document 2 includes a sensor for detecting the pressure of lubricant oil supplied by a lubricant pump to the engine. The lubricant pump is connected to the crank shaft of the engine through a pulley belt to be driven. Accordingly, as the engine speed is increased, the rotation speed of the lubricant pump increases to increase the pressure of the lubricant oil inside the engine. When a state that the engine speed is equal to or higher than a predetermined value while the pressure of the lubricant oil is lower than a reference pressure continues for a predetermined time period, the control device stops the engine or lowers the engine speed. Whereby, the dry operation of the engine is detected to lead to prevention of the engine seizing. Specifically, the state that the pressure of the lubricant oil is low while on the other hand the engine speed is high means that the supply amount of the lubricant oil is insufficient and lubrication insufficiency is caused. Therefore, this state is detected.
In the case where this dry operation detection control is applied to the aforementioned hydraulic power unit, when the rotation speed and the discharge pressure of the hydraulic pump fall within the hatched range in FIG. 4 (a range equal to or higher than the predetermined rotation speed and equal to or lower than the predetermined pressure), the hydraulic pump is stopped or lowered in its rotation speed. In other words, according to this state, in which the discharge pressure of the hydraulic pump is excessively low, while on the other hand the rotation speed of the hydraulic pump is high, it is detected that that the amount of the working oil sucked is insufficient and the hydraulic pump is in the dry operation state.    Patent Document 1: Japanese Unexamined Patent Application Publication 2006-214510    Patent Document 2: Japanese Unexamined Patent Application Publication 2003-172115